Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing and/or sensing electrode configurations. In the unipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a single electrode carried by the lead, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In the bipolar configuration, the pacing stimulation pulses are applied or intrinsic responses are sensed between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, with the most proximal electrode serving as the anode and the most distal electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to induce a depolarization of that chamber and this is followed by a mechanical contraction of that chamber when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses in one chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode. Recently, there has been the introduction of pacing systems that stimulate in multiple sites in the same chamber or in both the right and left ventricles or atria. These are termed multisite stimulation systems. Whenever we refer to dual chamber pacing, it will be inferred that multisite systems are included.
The energies of the applied pacing pulses must be above the pacing energy stimulation or capture threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize. If an applied pacing pulse has an energy below the capture threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the capture threshold.
Capture thresholds are assessed at periodic follow-up visits with the physician and the output of the pacemaker may be adjusted (programmed) to a safety margin that is appropriate based on the results of that evaluation. However, capture thresholds may change between scheduled follow-up visits with the physician. A refinement of the technique of periodic capture threshold measurement by the physician is the beat-by-beat monitoring of capture, delivery of a higher output back-up pulse when there is failure to recognize capture and automatic performance of capture threshold assessment and the automatic adjustment of the output of the pulse generator. This entire process is termed autocapture.
As is well known in the art, the capture threshold of a heart chamber can, for various reasons, change over time. Hence, pacemakers that incorporate autocapture are generally able to periodically and automatically perform autocapture tests. In this way, the variations or changes in capture threshold can be accommodated.
When a pacing pulse is effective in causing depolarization of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization of the heart muscle, it is referred to as “lack of capture”, “loss of capture” or “non-capture” of the heart. These terms should be considered synonyms and will be used interchangeably in this discussion.
In one known autocapture test, the pulse generator applies a succession of primary pacing pulses to the heart at a basic rate. To assess the threshold, the output of the primary pulse is progressively reduced. The output of each successive pair of primary pacing pulses is reduced by a known amount and capture is verified following each pulse. If a primary pulse results in loss of capture, a higher output backup or safety pulse is applied to sustain heart activity. If two consecutive primary pulses at the same output level result in loss of capture, the system identifies that output as being below the threshold and then starts to increment the output of the primary pulse. The output of successive primary pacing pulses is then incrementally increased until a primary pacing pulse regains capture. The output of the primary pulse which regains capture is the capture threshold to which a working margin is added in determining the pacing energy.
Delivery of a back-up pulse is normally provided about 60-100 milliseconds after the primary pulse which failed to capture the heart tissue. This effectively lengthens the normal AV interval by the 60-100 milliseconds. As used herein the term “AV interval” is meant to refer to the time interval beginning with an atrial event, either an atrial pacing pulse or an intrinsic P wave, and ending with the next scheduled ventricular pacing pulse. Hence, when the heart fails to respond to a primary pacing pulse with an evoked response, the normal AV interval of, for example, 150-250 milliseconds is extended for delivery of the back-up pulse 60-100 milliseconds later. Therefore, as far as the heart is concerned, the effective lengthened AV interval can be, for example, a minimum of 210 milliseconds and a maximum of 350 milliseconds with the exemplary interval ranges previously mentioned. During the extended functional AV interval, between an atrial event and the delivered back-up ventricular output, the atrium may have recovered on a physiologic basis to allow retrograde conduction to occur and the initiation of a pacemaker mediated tachycardia (PMT).
Another condition which may occur during autocapture is a fusion beat. A fusion beat occurs when a paced evoked response occurs essentially simultaneously with an intrinsic R wave. The result may be an attenuation of the evoked response signal amplitude to a value that is below the ER Sensitivity setting. If this happens, fusion which is associated with a myocardial depolarization will not be recognized and will be labeled “loss of capture.” There is an algorithm designed to screen for fusion since the presence of fusion implies intact AV nodal conduction. On the cycle following that first loss of capture associated with the primary pulse, the AV interval for the next cardiac cycle is lengthened by 100 to 120 ms. If conduction is intact, this extended AV delay will allow conduction to occur, the conducted QRS complex to be sensed and the ventricular output to be inhibited. This lengthened AV interval may also allow the atria to physiologically recover to allow a retrograde P wave to occur and the initiation of a PMT on the “fusion avoidance” cycle of the AutoCapture algorithm. A PMT results when the device detects a P-wave induced by retrograde conduction following either a native or paced ventricular complex in the atrial alert period, namely after completion of the Post-Ventricular Atrial Refractory Period (PVARP). When this occurs, the pacemaker subsequently, after a sensed AV interval and possible extension of that interval associated with the maximum tracking rate timing circuit, initiates a paced ventricular beat. Repeated stimulation at a high rate is sustained by heart tissue retrograde conduction and by pacemaker anterograde conduction.
Methods for preventing PMT are well known in the art. One such known method involves the use of programmable post-ventricular atrial refractory periods (PVARP), where the PVARP is programmed to be longer than the retrograde conduction interval. The downside of a long PVARP is that it limits the maximum allowed pacing rate. Another known method is based on the fact that the majority of PMTs are initiated by ventricular premature beats defined as an intrinsic ventricular event that is not preceded by an atrial beat. Thus, in this method, a ventricular premature beat causes a prolonged PVARP while the PVARP in other circumstance can be short thus allowing a higher maximum tracking rate. Still another method is to trigger a simultaneous atrial stimulation with a ventricular premature beat causing the atrium to be refractory precluding retrograde conduction from occurring. While the foregoing preventive measures are appropriate to prevent most PMTs triggered by a premature ventricular contraction or atrial undersensing as the conducted QRS will also be labeled a PVC. However, atrial undersensing followed by delivery of an atrial output with functional loss of atrial capture, true loss of atrial capture, upper rate behavior with sensed AV interval extension and other unique situations which may allow for retrograde conduction and thus precipitate a PMT during autocapture will not be able to handled by the unique PVC algorithms. One of these unique situations is true loss of ventricular capture associated with the primary pulse resulting in a functional extension of the AV delay created by delivery of the back-up pulse associated with the AutoCapture algorithm. The present invention addresses these issues.